1. Field of the Invention
This invention relates to motorized cameras and particularly to automatic camera rewind devices which automatically start to rewind the film when the film feed is stopped during winding.
2. Description of the Prior Art
Known automatic rewind devices use torque sensors to detect the changes in torque acting on a motor driven sprocket or spool when a film cartridge no longer feeds film, and change the winding operation to a rewinding operation.
Such devices have a number of problems. Specifically, they require the use of torque detectors capable of detecting torque changes. Such torque detectors are generally in the form of a resilient member such as a spring which is arranged to be deformed when the magnitude of the torque or force exceeds a predetermined level to indicate changes in torque. Therefore, the magnitude of the forces or torque detected vary depending upon the characteristics of the individual springs. The force necessary to initiate a rewinding operation may therefore vary from article to article. Hence, sufficient reliability cannot be assured.
Furthermore, the force necessary to extract the film from the cartridge varies from cartridge to cartridge, and even with temperature, because the film often hardens at low temperatures. Therefore, it is very difficult to assure automatic initiation of the rewinding operation in all situations.
Attempts have been made to eliminate the disadvantage by sensing the movement of the perforations of the film to detect whether the film is moving. This attempt is disclosed in U.S. Pat. No. 4,304,480. However, such a device requires detecting movement of the perforations of the film in combination with a current supply allowing the motor to be energized for a prescribed short period each time a winding operation is initiated. The arrangement is such that when it detects movement of the film perforation before the current supplied to the motor stops, termination of current to the motor is deferred to wind the film through one frame. Otherwise, the current supplied to the motor ends in that period. Therefore, the entire circuitry of the device becomes objectionably complicated. Another disadvantage of this device is that stoppage of the film feed before the film is wound a full frame does not cause automatic initiation of the film rewind operation.